bpy: how do I list the "pivot center for rotation/scaling" items?
Window manager context operators

Just as there is a window manager operator to call a pie menu, there is also a cluster of bpy.ops.wm.context_blah(...) operators, including

bpy.ops.context_pie_enum(data_path="")

which  expands the enum property elements in a pie menu.  The data_path parameter is the path from context. In this example  context.space_data.pivot_point the RNA string path from context is "space_data.pivot_point"



Test script, needs the context of a VIEW3D space. For convenience I prepended a button to the VIEW3D header. 

import bpy

def piebut(self, context):
    layout = self.layout
    op = layout.operator("wm.context_pie_enum")
    op.data_path = "space_data.pivot_point"

def register():
    bpy.types.VIEW3D_HT_header.prepend(piebut)

def unregister():
    bpy.types.VIEW3D_HT_header.remove(piebut)


if __name__ == "__main__":
    register()